The present invention relates to a method and equipment for measuring the properties of subsurface rock from a module close to the drilling bit operating during drilling. In particular it relates to a method and equipment for measuring such properties as the response time and amplitude of the electrokinetic response and deducing the pore fluid electrical conductivity, porosity and permeability of fluid-bearing porous rocks.
The measurement of permeability of rocks surrounding a borehole is important in assessing the location of water or oil reserves, including the quality and quantity of the reservoir rock. Existing methods are unable to measure the permeability of a porous rock directly with any accuracy from a downhole tool. It is valuable to measure the properties of a formation during drilling usually called logging while drilling (LWD) in order to vary the drilling as a response (called geosteering).
In addition to its value in the assessment of the quality and distribution of porous rock containing water or oil in reservoirs, the rock permeability is very important in determining at what rate and at what cost these fluids can be produced from production wells.
U.S. Pat. No. 3,599,085 describes a method in which a sonic source is lowered down a borehole and used to emit low frequency sound waves. Electrokinetic effects in the surrounding fluid-bearing rock cause an oscillating electric field in this and is measured at at least two locations close to the source by contact pads touching the borehole wall. The electromagnetic skin depth is calculated from the ratio of electrical potentials and the permeability of the rock deduced. U.S. Pat. No. 4,427,944 and the equivalent European Patent 0043768 describe a method which injects fluid at high pressure from a downhole tool to generate electrokinetic potentials; these are measured by contact electrodes against the borehole wall. The risetime of the electrical response is measured and from this the permeability of the porous rock is determined.
UK Patent 2,226,886A and the equivalent U.S. Pat. No. 4,904,942 describe several arrangements for recording electrokinetic signals from subsurface rocks mainly with the electrodes for measuring the signals at or close to the earth""s surface but including use of an acoustic source mounted on a downhole tool. There is no indication of permeability being deduced or of inferring porosity. A further related (inverse) method is described in European Patent 0512756A1, which contains several arrangements for setting out electrical sources and acoustic receivers (geophones) in order to measure electro-osmotic signals induced in subsurface rocks.
PCT Patent WO 94/28441 describes a method whereby sound waves of fixed frequency are emitted from a downhole source and the resulting electrokinetic potentials measured. An electrical source of fixed frequency is then used to produce electro-osmotic signals and the acoustic response measured. Using both responses together, the permeability is then deduced, provided the electrical conductivity of the rock is also separately measured.
In these methods the seismic shock is generated downhole at intervals and require a separate means for generating the signals downhole.
We have now devised an improved method for measuring the properties of porous rocks.
According to the invention there is provided a method for determining properties of rocks surrounding a borehole in which method electrokinetic signals generated in the rocks by the vibration induced by the drilling of the bore hole or by a seismic source emitting continuous pressure waves are detected by a detection means down the borehole and the signals processed to measure the porous rock properties.
Properties which can be measured by the method of the present invention include permeability, porosity and fluid properties such as viscosity, electrical conductivity and zeta potential.
The vibration signal produced by the drilling or by a seismic source is propagated outwards in all directions through the borehole fluid (the fluid in the borehole e.g. drilling mud etc.) and, subject to distortion by the borehole wall and variations in the rock, the signal propagates outwards substantially uniformly in all directions. The electrical signal generated within the surrounding rock is received and detected at the tool within the borehole from substantially all directions.
This invention also provides apparatus for measuring the properties of rocks surrounding a borehole, which apparatus comprises a module adapted to be lowered down a borehole in which module there is a means adapted to detect electrical signals generated in the rocks either by the seismic effect of the drilling of the borehole or by seismic signal emitted from the module.
The electrical signals can be detected by means of a pair of electrodes positioned within the borehole close to the borehole wall or, preferably, an electrode pair or short dipole antenna mounted downhole and positioned centrally and aligned axially within the borehole. In the equipment of the present invention it is convenient to use one or two electrical receivers spaced apart and preferably aligned vertically or horizontally at the centre of the borehole.
The electrical receiver preferably consists of one or two pairs of electrodes forming a short dipole antenna with electrically isolated ends. For each pair the electrodes are preferably connected to an amplifier which amplifies the signals whilst keeping them electrically isolated; this is carried out by referring the potential of each electrode independently to a floating reference potential. The signals are preferably amplified and converted to digital form before being communicated (e.g. by acoustic means) to the surface for recording and processing.
Preferably the means for detecting the electrical signals compares the potential at the ends of dipole antenna are compared by connecting them to an amplifier in which the potentials are preferably referred to a non-earthed reference (a virtual earth) and these new potentials are amplified and compared. Such a procedure allows amplification with very little distortion of the potential to be measured and with a high degree of common-mode noise rejection and is superior to other conventional methods of amplification.
Preferably the non-earthed reference potential is that of a common line in the amplification and data acquisition circuitry of the receiver and is not connected directly to earth.